1. Field of the Invention
The invention relates to an improved reusable flexographic printing sleeve.
2. Related Art
Printing sleeves are used in the flexographic printing process. In the original process, flexible printing plates were mounted to cylindrical mandrels to support the plates in a printing press. Due to the time consuming and sometimes difficult task of removing the flexible plates from the mandrel to remount new plates for different jobs, a cylindrical flexible base or sleeve was fabricated to slide over the mandrel and the printing plates were attached thereto as disclosed in U.S. Pat. No. 3,146,709 (Sep. 1, 1964) issued to Waldo E. Bass. In this way, sleeves could be removed from the mandrel quickly and a new sleeve with different printing plates could be mounted. Use of the sleeve had two (2) advantages, (1) a series of sleeves could be made ready to mount over a mandrel for quickly setting up successive runs of the press, and (2) the number of expensive mandrels needed to make successive runs was substantially reduced.
One problem with the use of sleeves was the difficulty associated with removal from the mandrel caused by friction between the metal mandrel and the plastic innermost layer of the sleeve. Bass (U.S. Pat. No. 3,146,709) disclosed a method of providing quick and easy removal of a sleeve from a mandrel with the use of a hollow mandrel having a pattern of apertures formed therein, through which air under pressure was passed to impinge upon the sleeve. The air pressure created a thin cushion of lubricating air between the mandrel and the sleeve allowing the sleeve to slide easily on and off of the mandrel. When the air pressure was reduced to ambient, the sleeve would frictionally bind itself securely around the mandrel.
Bass disclosed a sleeve made from a sheet of polyester plastic material having its edges on a bias and in abutting relation to each other. The cylindrical shape was disclosed as being maintained by helically winding a tape or strip of material with pressure sensitive adhesive, around the outside of the cylindrical body. A second layer, made of tape, was disclosed as helically wound about the first layer. The flexible printing plates with "sticky back" could then be mounted on the sleeve.
One problem with this type of sleeve was its propensity to allow air to pass through the sleeve by way of air gaps formed at the junction of the biased edges of the polyester plastic inner layer, which are not fully sealed by the helically wound paper strips and tape. Air leakage reduces the beneficial effect of the cushion of air on the one hand, and can adversly impinge upon and affect the mounted printing plates on the other. Another problem with the Bass sleeve is the "tear drop" shape it assumes when not in use.
As discussed in U.S. Pat. No. 3,978,254 issued Aug. 31, 1976 to Hoexter, and assigned to Mosstype Corporation (Mosstype), sleeves used at the time of the Bass disclosure were formed by joining the abutting ends of a rectangular sheet of film material over which plastic tape was helically wound. A second layer of plastic tape was then helically wound in the opposite direction over the first. Air gaps, though reduced in size, still existed with this type of sleeve. The "tear drop" shape remained a problem when the sleeve was not in use on a mandrel.
A further limitation upon the use of inner layers formed by one sheet rectangles or parallelograms, was that any length or diameter tube could not be readily manufactured without cutting the sleeve material to a specific size.
The Mosstype patent disclosed a sleeve formed by three (3) interlaiminated layers, each consisting of helically wound plastic tape. Two (2) of the helices are shown as wound in the same direction but offset axially relative to each other, i.e. "phased displaced", so that the air gap of the first layer is covered by the second layer. The third helix was shown as formed by winding the tape in the opposite direction of the first two layers. While this sleeve has the advantage of preventing air seepage therethrough, the gaps inherent in the winding of numerous revolutions of tape is apparent, and while the second two layers prevent air from impinging directly upon the printing plates, air forced into the gaps of the first and second layers can impinge directly upon the third layer and indirectly upon the attached printing plates. Another disadvantage of sleeves made by this method is their tendency to twist along the helix angle during a run of the press. Even a slight twisting action can cause the resulting print to be unacceptably distorted.